Body implantable electrical leads form the electrical connection between a pulse generator, such as a cardiac pacemaker, and body tissue, such as that of the heart, which is to be electrically stimulated. As is well known, the leads connecting pacemakers with the heart may be used for pacing or for sensing electrical signals produced by the heart, or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal tip an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end carrying an electrical connector assembly adapted to be received by a receptacle in the pacemaker. A flexible cable or coil conductor surrounded by an insulating sheath couples a terminal contact on the electrical connector assembly with the electrode at the distal tip.
To prevent displacement or dislodgment of the tip electrode and to maintain the necessary stable electrical contact between the tip electrode and the endocardial tissue, the electrode must be firmly anchored relative to the tissue. To achieve this, one type of lead, sometimes referred to as an active fixation lead, includes a pointed, extendable/retractable helix adapted to be screwed into the heart tissue to be stimulated. In this fashion, the position of the tip electrode is mechanically stabilized by positively anchoring the lead tip so that it remains securely in place during the lifetime of the implant.
The fixation helix may itself comprise the tip electrode in which case it is electrically coupled by means of a coil conductor to a rotatable terminal contact pin on the connector assembly. Rotational torque applied to the connector pin at the proximal end of the lead is transmitted via the coil conductor to the helix electrode which is thereby screwed into the heart tissue. Removal of the screw-in electrode from the endocardium is effected by counterrotation of the connector pin. Thus, in lead having a screw-in helix electrode, the coil conductor is used not only as a conductor for electrically coupling the connector pin and the helix electrode, but also as a tool for extending or retracting the helix electrode relative to the distal end of the lead during lead fixation or removal by rotating the connector pin. Whether the screw-in helix is electrically active or not, the degree of extension of the helix relative to the lead tip must be easily verified by the implanting physician.
Today's active fixation leads may be generally divided into two categories. Leads in the first category use a non-electrically conductive polymer header to retain the extendable/retractable helix while leads in the second category use a metal header for that purpose. Because of the fluoroscopic transparency of non-conductive polymers, leads of the first kind facilitate the confirmation of the degree of helix extension and retraction. In aid of such visual confirmation, a high-density, radiopaque metal ring is typically attached to the distal end of the polymer header. The ring thus serves as a fluoroscopic helix position marker; it is not electrically active to transfer electrical signals to and from the pacemaker to which the lead is connected.
Leads of the second category have high density metal headers that are not fluoroscopically transparent thus hindering visual confirmation of the degree of helix extension or retraction. Nevertheless, metal headers do have advantages. For example, their electrical conductivity allows cardiac signals generated in the cardiac tissue adjacent to the distal tip of the lead to be sensed for purposes of mapping localized heart activity. In addition, metal headers have greater strength than plastic headers and may be easily machined to form thin but strong header walls thereby making possible leads with small diameter distal ends.
Accordingly, there remains a need for a screw-in type of lead comprising a header that has the electrical and mechanical advantages of metal headers while at the same time allowing for the easy, visual fluoroscopic confirmation of helix extension and retraction afforded by non-conductive polymer headers.